Behind the Candelabra: A Facile Flame Vapor Deposition Method for Interfacial Engineering of Garnet Electrolyte To Enable Ultralong Cycling Solid-State Li-FeF Conversion Batteries.

The frustrating interfacial issue between Li metal anode and solid electrolyte is the main obstacle that restricts the commercial promotion of solid-state batteries. The garnet-type ceramic electrolyte with high stability against metallic Li has drawn much attention, but it also suffers from huge interfacial resistance and Li dendrite penetration due to the unavoidable formation of the carbonate passivation layer and limited interface contact. Herein, we propose a facile and effective method of flame vapor deposition to spray candle soot (CS) coating on the garnet surface. It enables the reduction of the carbonate layer and the conversion to a highly lithiophilic interlayer especially when in contact with molten Li. The lithiophilicity is rooted in the enrichment of graphitic polycrystalline domains in CS, which can be chemically or electrochemically lithiated to form the ionic/electronic dual-conductive network containing LiC moieties. The CS interlayer binds the Li metal with the garnet electrolyte tightly with gradual transition of Li-ion conductivity, leading to a significant reduction of the area-specific resistance to 50 Ω cm at 60 °C with high cycling and current endurance. Garnet-based symmetric cells and solid-state full cells conducting this strategy exhibit impressive electrochemical reversibility and durability under the preservation of the compact interface and smooth Li plating/stripping. The modified Li/garnet/FeF batteries exhibit a discharge capacity as high as 500 mA h g and long-term cyclability for at least 1500 cycles (with capacity preserved at 281.7 and 201 mA h g at 100 and 200 μA cm, respectively). This candle combustion strategy can be extended to more ceramic electrolytes compatible with high-temperature pretreatment.